Underlying

原資産。オプションの基礎となる株式・ETF・指数で、その価格がオプションの価値を最終的に左右する。

The underlying is the asset your option is built on, most often a stock, an ETF or an index. When you buy a call on Apple, Apple shares are the underlying, and the option's value moves because the underlying moves. Everything else about the contract, from strike to expiry, only matters in relation to where that underlying price sits.

In practice you watch the underlying first and the option second. A standard equity option controls 100 shares, so one AAPL contract gives you exposure to 100 Apple shares. If the underlying trades at 190 and you hold a 200 strike call, you already know the option is out of the money and needs a real move up to gain intrinsic value.

A common mistake is treating the option in isolation and forgetting that dividends, earnings dates and gaps happen at the underlying level. An overnight gap in the stock can change your position long before you get a chance to react, which is why traders check what the underlying is doing around scheduled events.

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